An investigation of structure-function relationships in occluding junctions and cellular control of their maintenance and modulation will continue. Mammary epithelial cells grown on plastic or on hydrated collagen gels will be exposed to a series of agents reported or expected to modify junction permeability; transepithelial resistance of gel-grown cultures mounted on Ussing chambers will be measured, and morphology of the junction and underlying cortical cytoplasm will be examined by electron microscopy.